(1) Field of the Invention
The present invention relates to a process and an apparatus for producing single crystals of oxides.
(2) Related Art Statement
A single crystal of lithium potassium niobate and a single crystal of lithium potassium niobate-lithium potassium tantalate solid solution are remarked especially as single crystals for a blue light second harmonic generation (SHG) device for a semiconductor laser. The device can emit even the ultraviolet lights having the wavelengths of 390 nm or so, thus the crystals can be suitable for wide applications such as optical disk memory, medicine and photochemical fields, and various optical measurements by using such short-wavelength lights. Since the above single crystals have a large electro-optic effect, they can be also applied to optical memory devices using the photo-refractive effect.
However, for an application of a second harmonic generation device, for example, even a small fluctuation in a composition of the single crystal may affect the wavelength of the second harmonic wave generated by the device. Therefore, the specification of the range of the composition required for the single crystals is severe, and the fluctuation in the composition should be suppressed in a narrow range. However, since the composition consists of as many as three or four components, growing a single crystal at a high rate is extremely difficult to achieve while controlling the proportions of the components to be constant.
In addition, laser beams, having a short wavelength of around 400 nm, for example, need to be propagated inside of the single crystal at as high an output density as possible in the case of the optical applications, particularly the second harmonic generation. Further, optical damage needs to be suppressed. The good crystallinity of the single crystal is required for this purpose.
NGK Insulators, Ltd. suggested a micro (xcexc) pulling-down method for growing the above single crystal with the constant compositional proportions, for example, in JP-A-8-319,191. In this method, a raw material comprising lithium potassium niobate is put into a platinum crucible and melted, and then the melt is pulled down gradually and continuously through a nozzle attached to the bottom of the crucible.
Such a micro pulling-down method is useful for growing the above oxide single crystal having many components. However, for widespread uses of the above single crystal, increased industrial productivity is indispensable. For this purpose, the capacity of the crucible needs to be increased to grow as many single crystals as possible. To attain this, it is necessary that a driving unit is provided under the crucible, a seed crystal is bonded to an upper end of the driving unit, the seed crystal is contacted with the melt in the crucible, and the seed crystal is pulled down as straight as possible at a high precision. As such a high precision pulling-down mechanism, a driving unit using a rail may be used, for example.
However, inventors"" mass production experiments revealed that a single crystal having good crystallinity was successfully grown by pulling down the seed crystal at a high precision in the beginning, but as the grown length of the oxide single crystal increases (with lapse of time), the crystallinity of the oxide single crystal is deteriorate to reduce the yield.
It is an object of the present invention to prevent deterioration in the crystallinity of a single crystal which would accompany increase in the grown length of the single crystal, when a raw material for the oxide single crystal is melted in a crucible, a seed crystal is contacted with the melt, and the oxide single crystal is grown, while being pulled down through an opening of the crucible in a given pulling-down axis.
A first aspect of the present invention relates to a process for producing an oxide single crystal, comprising the steps of:
melting a raw material for a single crystal of an oxide inside a crucible, contacting a seed crystal with the resulting melt, growing the oxide single crystal by pulling-down the melt through an opening of the crucible in a given pulling-down axis, fixedly holding the seed crystal and then reducing an angle of a given crystalline orientation of the seed crystal selected for growing the single crystal to the pulling-down axis.
A second aspect of the present invention relates to an apparatus for producing a single crystal of an oxide, comprising a crucible for melting a raw material of the oxide single crystal, and a stand for holding a seed crystal, said stand comprising a holding member for fixedly holding the seed crystal, and an inclining mechanism attached to the under side of the holding member, and adapted to incline an angle of the holding member to a given pulling-down axis, wherein a raw material of the oxide single crystal is melted in the crucible, the seed crystal is contacted with the resulting melt, the oxide single crystal is grown by pulling-down the melt through an opening of the crucible, and an angle of the holding member to the pulling-down direction is reduced.
With respect to the above-mentioned cases where the crystallinity of the single crystals were gradually deteriorated as the grown length thereof increased, the inventors observed the state of the single crystal fibers and the plates in detail. As a result, it was found that the diameter slightly decreased in a direction from one end to the other of the single crystal fiber. In other words, there was tendency that the diameter of the single crystal fiber slightly decreased in a direction from a growth-starting end to a growth-terminating end. Further, it was found that the single crystal plate slightly twisted.
Based on such knowledge, the inventors further made investigations, and then considered that a way of holding the seed crystal might be a cause. That is, the seed crystal needs to be pulled down after being bonded to the holding member. In order to bond the seed crystal to the holding member, a specific crystalline orientation of the seed crystal is selected as a crystal-growing direction, and then this crystalline orientation is aligned with the direction of the pulling-down axis. The thus selected crystalline orientation is ordinarily in parallel to the direction of a specific side constituting the outer configuration of the seed crystal. Therefore, if the seed crystal is bonded to the holding member just straightly with this specific side being taken as a reference, the direction of the specific side of the seed crystal (that is, the above crystalline orientation of the seed crystal) must be aligned with the pulling-down axis.
However, even if the seed crystal is bonded to the holding member in the state that the above crystalline orientation of the seed crystal is aligned with the pulling-down axis, it is considered that the crystalline orientation of the seed crystal may actually deviated from the pulling-down axis owing to unspecified causes such as non-uniform shrinkage of the adhesive during curing. In such a case, even if the seed crystal is pulled down in the direction of the pulling-down direction, the crystalline orientation of the seed crystal is oblique to the pulling-down axis. The oxide single crystal grows in the direction of the crystalline orientation of the seed crystal. Accordingly, the single crystal-growing direction is slightly inclined to the pulling-down axis. As a result, it is considered that although no serious problem exists at a time when the single crystal begins to grow, the diameter of the single crystal fiber gradually decreases or the width of the single crystal plate gradually decreases as the single crystal grows long.
Based on this hypothesis, the inventors tried to provide an inclining mechanism for inclining the holding member by varying its angle to the pulling-down axis, incline the holding member relative to the pulling-down axis after the seed crystal is held by the holding member, and thereby reduce the angle of the pulling-down axis to the crystalline orientation selected for the growth of the crystal. As a result, the inventors succeeded in preventing the shrinkage of the diameter of the single crystal fiber and the shrinkage of the width of the single crystal plate. In addition, the inventors discovered that the crystallinity of the single crystal is not deteriorated as compared with that at the time of starting the crystal growth.
In the present invention, the angle of the crystalline orientation of the seed crystal selected for the crystal growth to the pulling-down axis is particularly preferably controlled to not more than 0.5xc2x0, more preferably not more than 0.2xc2x0.
The seed crystal preferably a cross-sectional shape identical with or similar to but smaller than or anyway reduced in size in any direction from that of the bottom face of the nozzle portion. In other words, if the bottom of the nozzle portion is of a polygonal cross-sectional shape, for example, a rectangular cross-sectional shape, the seed crystal has a polygonal, e.g., rectangular cross-sectional shape identical with or similar to but smaller than or reduced in size from that of the nozzle bottom face. As to the last case, if the bottom face of the nozzle portion is 50 mm widexc3x972 mm width, the cross-sectional shape of the seed crystal may be 30 mmxc3x971, for example.
In a preferred embodiment of the present invention, the stand is provided with a turning mechanism for turning the holding member around the pulling-down axis. After the seed crystal is held by the holding member, the holding member is turned to match the cross-sectional shape of the seed crystal with the bottom face of the nozzle portion. xe2x80x9cTo match the cross-sectional shape of the seed crystal with the bottom face of the nozzle portionxe2x80x9d means that the sides of the cross-sectional shape of the seed crystal are arranged to be matched with or faced with corresponding ones of the bottom face of the nozzle portion. By turning the seed crystal around the pulling-down axis, angles defined by plural pairs of sides of the polygonal cross-sectional shape of the seed crystal and corresponding ones of the polygonal shape of the opening of the crucible, respectively, are reduced. This angle is preferably not more than 0.5xc2x0, more preferably not more than 0.3xc2x0.
This embodiment will be explained. For example, when a single crystal plate is grown, the grown single crystal plate is slightly twisted besides the above-mentioned problems. As mentioned above, the present inventors tried to provide the turning mechanism for turning the seed crystal around the pulling-down axis and approach the configuration of the cross section of the seed crystal to that of the bottom face of the nozzle by turning the holding member. As a result, it was discovered that the twisting deformation of the single crystal plate grown was reduced or almost not observed and that the crystallinity was improved over the entire length of the single crystal plate.
The configuration of the bottom face of the nozzle portion and the cross-sectional configuration of the seed crystal are particularly square or rectangular. In that case, the ratio between the adjacent sides particularly preferably ranges 1:1 to 100.